Great article. I joined the dark side this year and bought a 2010 KTM 450SXF. It is fairly mellow of the bottom and then revs out making more and more power, so it is pretty easy to go fast on. In a straight line with traction, my KX500 would eat it alive. And though the KTM is pretty stable, it isn't as stable at speed as a KX500.
But, on a tight track, with ruts or slippery corners, the KTM is waaaay easier to get around on. I love 'em both, but time marches on; the KTM is a better motocross race machine. Without mods I'm not sure the 450 could run with a KX500 off road.